A typical prior art image reading apparatus has such a structure as shown in FIG. 11. The illustrated image reading apparatus comprises a substrate 92 provided with a plurality of light sources 91, a lens array 93, a plurality of light receiving elements 94 and a case 90 for housing these components. The case 90 has an upper surface on which a transparent plate 97 is mounted. The transparent plate 97 has an obverse surface a portion of which serves as an image read line Sa. As shown in FIG. 12, the light sources 91 are arranged in a row. The case 90 includes a hollow portion 96 for guiding light emitted from the light sources 91 toward the image read line Sa.
In this image reading apparatus, when light emitted from the light sources 91 impinges on the image read line Sa, the light is reflected by the document D. The reflected light is collected by the lenses of the lens array 93 to be received by the light receiving elements 94. Each of the light receiving elements 94 outputs signals corresponding to the received amount of light. Thus, an image of the document D is read by line by line.
In an image reading apparatus, an image read line Sa needs to be illuminated as uniformly as possible along its entire length for enhancing the image reading quality. However, the above-described prior art structure has a problem that the image read line Sa cannot be illuminated uniformly, as will be described below.
In the prior art, the hollow portion 96 simply extends along the row of light sources 91. Therefore, light emitted from the light sources 91 overlaps at a longitudinally central portion of the image read line Sa. On the other hand, light does not overlap at longitudinally opposite ends of the image read line Sa, because only a small number of light sources are arranged in facing relationship to these ends of the image read line. As a result, the longitudinally opposite ends of the image read line Sa are less illuminated than the longitudinally central portion.
Further, in the prior art structure, the case 90 is made of black resin so that all the surfaces defining the hollow portion 96 are black for absorbing light. This is because black surfaces prevent light from scattering upon reflecting near the light receiving elements 94, so that scattering reflection light will not enter the light receiving elements 94. In the prior art structure, therefore, light impinging on two surfaces 96a defining the longitudinally opposite ends of the hollow portion 96 is not reflected toward the image read line Sa. This also causes the longitudinally opposite ends of the image read line Sa to be less illuminated than the central portion.
Moreover, auxiliary elements (not shown) such as jumpers, capacitors, resistors and/or the like may be mounted on the obverse surface of the substrate 92. In such a case, the auxiliary elements may have light reflectivity which is different from that of the obverse surface of the substrate 92. On the other hand, all of light emitted from the light sources 91 does not directly reach the image read line Sa. Some of light is reflected at the obverse surface of the substrate 92 before reaching the image read line Sa. In the prior art structure, the auxiliary elements reflect light differently from the obverse surface of the substrate 92. As a result, portions of the image read line Sa corresponding to the auxiliary elements may be illuminated with different luminosity from the other portions of the image read line Sa.